The present invention relates to a waveguide array antenna, composed from a number of open waveguides lying in one plane on the radiation side, of which a part is fed on the reverse side thereof, while the other ones are closed off.
Antennas of this type are typically referred to as reactively loaded waveguide array antennas. The open waveguide ends of the antennas need not lie in a flat plane, a curved plane or a faceted plane is possible as well. The waveguides in the array have their open ends periodically positioned in such plane. Through a mutual coupling, the non-fed waveguides constitute separate radiation elements, while due to the interaction between these radiation elements, a directional beam can be realized. Said plane may also be materially present in the form of a plate of the same material as the waveguides. The waveguides are short-circuited on the reverse side by means of a closure. The length of the waveguides, i.e. the distance from the short-circuit to the open end of the waveguides, influences the phase adjustment of the wave pattern in the opening thereof and in that of adjacent waveguides and, accordingly, the direction of the beam to be transmitted. Since the present invention relates to a passive antenna, the same applies to the receiving situation as to the transmitting situation, on account of the reciprocity principle. Hereinbelow, only the transmitting situation will be referred to.
A waveguide array antenna as referred to above is known from, for instance, an article by F. Arndt et al.: Generalized Moment Method Analysis of Planar Reactively Loaded Rectangular Waveguide Arrays, IEEE Transactions on Antennas and Propagation, Vol. 37, No. 3, Mar. 1989, pp. 329-338. In particular in FIG. 6 of this article, it is indicated how an antenna beam widens if instead of all waveguide array elements, only one is fed. In the case of an array antenna having a fixed beam alignment, the feed of all waveguides is laborious, requiring a complicated distribution network which involves considerable losses in the array antenna, and renders the antenna rather voluminous, heavy and expensive. In the case of an array antenna having variable beam alignment, a phase adjustment of the separate waveguide radiation elements should be performed by means of mechanical or electronic phase shifters and a so-called beam steering computer. This, too, is an expensive solution. It is true that the feeding of only a few waveguides and the short-circuiting of the other waveguides meets these drawbacks, but the result is a considerably wider beam, as a consequence of which, when used in a radar apparatus, the accuracy in determining target coordinates and following a target by means of the radar beam becomes far less accurate. Further, the electromagnetic behavior of the two known types of array antennas is virtually identical.
The object of the invention is to prevent the above-mentioned drawbacks at least to a large extent and to provide a reactively loaded waveguide array antenna whereby a narrow beam can be obtained all the same.
To that end, in accordance with the invention, the waveguide array antenna as described in the preamble is characterized in that on the radiation side at least one two-layer structure is provided, integrated with the open waveguides and consisting of a dielectric layer and a perforated, electrically conductive layer, in which two-layer structure, together with the layer or the face on which this two-layer structure has been provided, a periodically repeating electromagnetic resonance pattern can be generated in parallel directions relative to said face, of which the energy, outputted via the perforations, forms a transmission beam. In this respect. the two-layer structure constitutes a so-called angular filter.
U.S. Pat. No. 4,169,268 discloses a waveguide array antenna wherein all open waveguides are fed on the reverse side thereof, so that the advantages of the array antenna in the article cited and of the array antenna according to the invention are not obtained. It is true that in this US patent, a number of two-layer structures are provided behind the antenna, but these two-layer structures are not integrated with the open waveguides. These two-layer structures are meant to be positioned in front of an antennaxe2x80x94array antenna, parabolic antenna or another type of antenna.
Further, in accordance with the invention, the open waveguides and the two-layer structure are integrated with each other by means of a matching structure present between the open waveguides and the two-layer structure. This realizes an adaptation of the waves guided through the waveguides to waves propagating in the spacexe2x80x94through the two-layer structurexe2x80x94and the other way round. Hence, there is an adaptation both to the angular filter and to the input structure formed by the waveguides.
By giving this matching structure frequency-filtering properties, the antenna behavior can further be influenced in a desired manner.
An efficient array antenna is obtained when the matching structure and the two-layer structure are surrounded, parallel to the main axis of the antennaxe2x80x94by electrically conductive limiting elements. Contrary to the US patent cited, where the two-layer structure is surrounded by radiation-absorbing material, when electrically conductive limiting elements, in particular plates, are used, all energy is radiated forwards. No energy losses caused by absorption occur; the limiting elements form, as it were, one large waveguide.
The perforations in the electrically conductive layer are equally spaced apart and aligned with corresponding waveguide openings. However, the number of perforations need not correspond to the number of waveguide openings.
The features mentioned boil down to the introduction of a so-called angular filter, i.e. a waveguide filter in the angular domain, while substantially energy is transmitted in an angular interval, defined by angular values relative to the center line of a beam to be transmitted. The direction of this center line is defined by the phase adjustment of the waveguides. Such angular filter can be designed as low-pass angular filter, in which case a beam can be obtained whose beam width may be considerably smaller than that of a waveguide array antenna without said two-layer structure acting as angular filter. Such angular filter can also be designed as angular band filter, in which case a conical beam having an annular section is obtained.
In particular, a narrow beam can be obtained when several two-layer structures are provided on top of each other, with the periodicity of the perforations in an electrically conductive layer of such two-layer structure being a fraction or a multiple of the periodicity in an electrically conductive layer of an adjoining two-layer structure and the dimensions of the perforations in adjacent electrically conductive layers differing per layer.